The invention relates in general to magnetic field sensors and more particularly to integrated magnetic field sensors formed on a single semiconductor die and requiring the use of a magnetic field for functions such as domain setting, calibration, establishing a bias or offset magnetic field, providing a feedback field, determining a sensor transfer characteristic, or other functions requiring a magnetic field.
Extremely small magnetic field sensing devices can be made using strips of a magnetoresistive film of a material such as Permalloy™. The magnetization of the film forms an angle with current flowing in the film and the resistance of the film varies with this angle. When the magnetization of the film is parallel to the current, the resistance is at a maximum, and when it is perpendicular to the current, the resistance has a minimum value.
Magnetic field sensing devices may be used in many applications including, but not limited to, magnetic signal or power isolation, solid state compassing, e.g., in automobiles; current measuring devices; signature detection, e.g., metal detection; and anomaly detection, e.g., position sensing.
Solid state compassing may be used in personal items, for example, in a watch. Position sensing may be used to sense the position of a medical device, such as a catheter within the body of a patient. These and other applications have created requirements for magnetic sensing devices that are of a smaller size and that require less power than the devices of the past.
The present invention also relates to magnetic field sensing devices and to their use in electrical current sensing and measurement applications. Various magnetic field sensing techniques have been used for the measurement of current. Current sensing can be accomplished using Hall Effect transducers. In one arrangement, an electromagnet having a coil that carries the current to be measured produces a magnetic field. A Hall device is used to sense the magnetic field which is proportional to the current. A pole piece may be used to concentrate the magnetic field where the Hall device is located. The Hall device may be designed to provide an analog output or a digital output. In another arrangement, the current to be measured is passed through a coil on a soft iron core having an air gap. A Hall device is placed in the air gap to the sense the magnetic field generated by the current. This technique can be refined by placing a second compensating coil carrying and adjustable and known current on the iron core that opposes the magnetic field created by the current to be measured. The Hall device then senses a condition when the field from the coil carrying the measured current has been nulled by the field from the compensating coil. The number of turns in each of the coils is used to relate the known current to the current to be measured. One disadvantage of the methods just described is that they require that the current measuring arrangement be inserted into the circuit carrying the current to be measured.
Non-contact clamp-on current measuring devices provide a convenient means for measuring DC and AC line current in a current carrying conductor without the need to interrupt the circuit to insert the measuring device. Present methods of non-contact current measurements in conductors often consist of the use of iron or other ferrous types of magnetic materials configured so as to surround or nearly surround a current carrying conductor in a transformer-like configuration. These present methods, sometimes referred to as a “current clamp” or “clamp-on current probe” are widely employed in measuring a.c. currents in wires and other electrical conductors. These devices provide a means of rapidly measuring the AC current by surrounding the conductor with a closed or nearly closed magnetic circuit which is configured as a transformer which is designed for a convenient ratio for measurement to provide, for example, one milliampere per ampere, or one millivolt per ampere. The output of these clamp-on current probes is then read out on a meter or attached via wires to a multimeter. Electronic displays may also be used to display current values. The use of magnetic materials and the need for these materials to surround the current carrying conductor has some disadvantages.
A second common non-contact method of measuring current utilizes the Hall Effect. A Hall element placed in the region of a magnetic field provides an output voltage proportional to the field. One known current transducer uses a Hall-effect device arranged in a gap of a toroidal core. The conductor carrying the current to be monitored is arranged to pass through the toroid. The Hall-effect element measures directly the flux resulting from the introduction of MMF in the toroidal core due to the current in the conductor. Hall element devices are often utilized for both AC and DC non-contact current measurements in wires and other conductors and are available from manufacturers and distributors of AC clamp-on current probes.
Certain current measuring devices that utilize magnetoresistive sensors and require electrical connection into the circuit being measured are also known. For example, in one arrangement a sensor is mounted on one side of a circuit board with permanent magnets mounted on the same side of the circuit board and near opposite edges of the sensor to provide a magnetic field for initial magnetic alignment and biasing of the sensor. A coil which carries the current to be measured is mounted on the other side of the circuit board and opposite the sensor. For low current measuring applications, the coil may be many turns of wire and for high current measuring applications, the coil may consist of a U-shaped heavy conductor with electrical connections made at the ends of the U. In this arrangement, the sensitivity of the current sensor depends on the magnet strength and location. The matching characteristics of the magnets have a great effect on the accuracy and linearity of the current sensor. The inability to attach magnets, for example by gluing, to achieve the same spacing and alignment in devices of this type will also affect the accuracy and linearity of the sensor. Thus, a need exists for a current measuring apparatus that does not require the use of ferrous materials to surround the conductor in which current is being measured or the use of ferrous materials to increase the flux density, and further provides wide flexibility in current measuring ranges.